protocol

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This file documents version 2 of the svn protocol.

1. Syntactic structure
----------------------

The Subversion protocol is specified in terms of the following
syntactic elements, specified using ABNF [RFC 2234]:

  item   = word / number / string / list
  word   = ALPHA *(ALPHA / DIGIT / "-") space
  number = 1*DIGIT space
  string = 1*DIGIT ":" *OCTET space
         ; digits give the byte count of the *OCTET portion
  list   = "(" space *item ")" space
  space  = 1*(SP / LF)

Here is an example item showing each of the syntactic elements:

  ( word 22 6:string ( sublist ) )

All items end with mandatory whitespace.  (In the above example, a
newline provides the terminating whitespace for the outer list.)  It
is possible to parse an item without knowing its type in advance.

Lists are not constrained to contain items of the same type.  Lists
can be used for tuples, optional tuples, or arrays.  A tuple is a list
expected to contain a fixed number of items, generally of differing
types.  An optional value is a list containing either zero or a fixed
number of items.  An array is a list containing zero or more items of
the same type.

Words are used for enumerated protocol values, while strings are used
for text or binary data of interest to the Subversion client or
server.  Words are case-sensitive.

For convenience, this specification will define prototypes for data
items using a syntax like:

  example: ( literal [ rev:number ] ( data:string ... ) )

A simple word such as "literal", with no colon, denotes a literal
word.  A choice of words may be given in braces with "|" separating
the choices.  "name:type" specifies a parameter with the given type.
A type is "word", "number", "string", "list", or the name of another
prototype.  Brackets denote an optional tuple.  Parentheses denote a
tuple, unless the parentheses contain ellipses, in which case the
parentheses denote an array containing zero or more elements matching
the prototype preceding the ellipses.  If a tuple has an optional part
after the fixed part, a '?' marks places where the tuple is allowed to
end.  The following tuple could contain one, three, or four or more
items:

  example: ( fixed:string ? opt1:number opt2:string ? opt3:number )

For extensibility, implementations must treat a list as matching a
prototype's tuple even if the list contains extra elements.  The extra
elements must be ignored.

In some cases, a prototype may need to match two different kinds of
data items.  This case will be written using "|" to separate the
alternatives; for example:

  example: ( first-kind rev:number )
         | second-kind

The "command response" prototype is used in several contexts of this
specification to indicate the success or failure of an operation.  It
is defined as follows:

  command-response: ( success params:list )
                  | ( failure ( err:error ... ) )
  error: ( apr-err:number message:string file:string line:number )

The interpretation of parameters in a successful command response is
context-dependent.

URLs and repository paths are represented as strings.  They should be in
canonical form when sent over the protocol.  However, as a matter of input
validation, an implementation should always canonicalize received paths if it
needs them in canonicalized form.

2. Connection establishment and protocol setup
----------------------------------------------

By default, the client connects to the server on port 3690.

Upon receiving a connection, the server sends a greeting, using a
command response whose parameters match the prototype:

  greeting: ( minver:number maxver:number mechs:list ( cap:word ... ) )

minver and maxver give the minimum and maximum Subversion protocol
versions supported by the server.  mechs is present for historical
reasons, and is ignored by the client.  The cap values give a list of
server capabilities (see section 2.1).

If the client does not support a protocol version within the specified
range, or does not support any of the specified SASL mechanisms, it
closes the connection.  Otherwise, the client responds to the greeting
with an item matching the prototype:

  response: ( version:number ( cap:word ... ) url:string )

version gives the protocol version selected by the client.  The cap
values give a list of client capabilities (see section 2.1).  url
gives the URL the client is accessing.

Upon receiving the client's response to the greeting, the server sends
an authentication request, which is a command response whose arguments
match the prototype:

  auth-request: ( ( mech:word ... ) realm:string )

The mech values give a list of SASL mechanisms supported by the
server.  The realm string is similar to an HTTP authentication realm
as defined in [RFC 2617]; it allows the server to indicate which of
several protection spaces the server wishes to authenticate in.  If
the mechanism list is empty, then no authentication is required and no
further action takes place as part of the authentication challenge;
otherwise, the client responds with a tuple matching the prototype:

  auth-response: ( mech:word [ token:string ] )

mech specifies the SASL mechanism and token, if present, gives the
"initial response" of the authentication exchange.  The client may
specify an empty mechanism to decline authentication; otherwise, upon
receiving the client's auth-response, the server sends a series of
challenges, each a tuple matching the prototype:

  challenge: ( step ( token:string ) )
           | ( failure ( message:string ) )
           | ( success [ token:string ] )

If the first word of the challenge is "step", then the token is
interpreted by the authentication mechanism, and the response token
transmitted to the server as a string.  The server then proceeds with
another challenge.  If the client wishes to abort the authentication
exchange, it may do so by closing the connection.

If the first word of the challenge is "success", the authentication is
successful.  If a token is provided, it should be interpreted by the
authentication mechanism, but there is no response.

If the first word of the challenge is "failure", the authentication
exchange is unsuccessful.  The client may then give up, or make
another auth-response and restart the authentication process.

RFC 2222 requires that a protocol profile define a service name for
the sake of the GSSAPI mechanism.  The service name for this protocol
is "svn".

After a successful authentication exchange, the server sends a command
response whose parameters match the prototype:

  repos-info: ( uuid:string repos-url:string )

uuid gives the universal unique identifier of the repository, and
repos-url gives the URL of the repository's root directory.  The
client can now begin sending commands from the main command set.

2.1 Capabilities

The following capabilities are currently defined:

  edit-pipeline     If both the client and server present this
                    capability, edits operations will use pipelining.
                    See section 3.1.2.

3. Commands
-----------

Commands match the prototypes:

  command: ( command-name:word params:list )

The interpretation of command parameters is different from command to
command.

Initially, the client initiates commands from the main command set,
and the server responds.  Some commands in the main command set can
temporarily change the set of commands which may be issued, or change
the flow of control so that the server issues commands and the client
responds.

Here are some miscellaneous prototypes used by the command sets:

  proplist:  ( ( name:string value:string ) ... )
  propdelta: ( ( name: string [ value:string ] ) ... )
  node-kind: none|file|dir|unknown
  bool:      true|false

3.1. Command Sets

There are three command sets: the main command set, the editor command
set, and the report command set.  Initially, the protocol begins in
the main command set with the client sending commands; some commands
can change the command set and possibly the direction of control.

3.1.1. Main Command Set

The main command set corresponds to the svn_ra interfaces.  After each
main command is issued by the client, the server sends an auth-request
as described in section 2.  (If no new authentication is required, the
auth-reuqest contains an empty mechanism list, and the server proceeds
immediately to sending the command response.)  Some commands include a
second place for auth-request point as noted below.

  get-latest-rev
    params:   ( )
    response: ( rev:number )

  get-dated-rev
    params:   ( date:string )